1. Field of the Invention
This invention relates to a method and apparatus for compressing an image signal representing a radiation image. This invention also relates to a method and apparatus for reproducing a radiation image from an image signal representing a radiation image, and more particularly to a method and apparatus for reproducing a radiation image from an image signal, which has been subjected to interpolation processing carried out for enlarging or reducing the radiation image.
2. Description of the Prior Art
Radiation images recorded on sheets of X-ray film have heretofore been read out by use of film digitizers, or the like, and electric image signals representing the radiation images are thereby obtained. Visible radiation images are then reproduced from the image signals and displayed on display devices, such as cathode ray tube (CRT) display devices.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as the human body, is stored on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet when it is exposed to the stimulating rays is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material, such as photographic film, on a display device, such as a cathode ray tube (CRT) display device, or the like.
When a radiation image of an object is recorded on a recording medium, such as X-ray film or a stimulable phosphor sheet, a grid is often located between the object and the recording medium such that radiation scattered by the object may not impinge upon the recording medium. The grid is constituted of bars of a radiation-impermeable material, such as lead, and bars of a radiation-permeable material, such as aluminum, which are alternately located in parallel at small pitches of approximately 4.0 bars/mm. When the grid is used during the recording of a radiation image of an object on a recording medium, radiation scattered by the object is prevented from impinging upon the recording medium, and therefore the contrast of the radiation image of the object can be kept high. However, a grid image having a striped pattern is recorded together with the object image on the recording medium.
In general, in radiation image read-out apparatuses, wherein an image signal is detected from a recording medium which has a radiation image recorded thereon, light which is emitted from the recording medium and which carries information about the radiation image is photoelectrically detected and converted into an image signal. The image signal is then sampled at sampling intervals of .DELTA.x=1/(2.multidot.fss) corresponding to the spatial frequency, which is the maximum of a spatial frequency range necessary for image information. The spatial frequency, which is the maximum of a spatial frequency range necessary for image information, is herein denoted by fss. The sampled image signal is then digitized. In cases where the radiation image comprises the object image and a grid image superposed upon the object image, the image signal obtained in the manner described above includes not only the information representing the radiation image of the object but also noise which is caused to occur by the grid image.
FIG. 6A is a graph showing the spatial frequency characteristics of a radiation image, which has been recorded on a recording medium and which comprises an object image and a grid image superposed upon the object image, in a direction along which the stripes of the striped pattern of the grid image stand side by side. By way of example, it is herein assumed that, during the recording of the radiation image, a grid having the bars of a radiation-impermeable material and the bars of a radiation-permeable material, which are alternately located in parallel at pitches of 4.0 bars/mm, was used. The spatial frequency of the grid image is 4 cycles/mm. Also, it is assumed herein that the spatial frequency fss, which is the maximum of a spatial frequency range necessary for the reproduction of a visible radiation image of the object, is 2.5 cycles/mm.
FIG. 6B is an explanatory graph showing how noise occurs when an image signal is sampled at sampling intervals of .DELTA.x=1/(2.multidot.fss)=0.2 (mm) corresponding to the spatial frequency fss=2.5 (cycles/mm), i.e. is sampled five times per mm. When such sampling intervals are applied, it is possible to obtain information in the spatial frequency region which is below the spatial frequency fss=2.5 (cycles/mm), which is the maximum of a spatial frequency range necessary for the reproduction of a visible radiation image of the object. In FIG. 6B, the same curve as that shown in FIG. 6A is indicated by the solid line. As indicated by the broken line, noise occurs at the position corresponding to 1 cycle/mm, with which the position of the peak occurring at 4 cycles/mm coincides when the curve indicated by the solid line is folded back from the part corresponding to fss=2.5 (cycles/mm). Such noise is referred to as "aliasing." Specifically, as indicated by the broken line in FIG. 6B, aliasing noise corresponding to a spatial frequency of 4 cycles/mm of the grid image occurs at the position corresponding to 1 cycle/mm.
FIG. 6C is a graph showing the spatial frequency characteristics of the radiation image represented by an image signal obtained from the sampling in which sampling intervals of .DELTA.x=1/(2.multidot.fss)=0.2 (mm) are applied. As illustrated in FIG. 6C, the image signal includes the noise corresponding to the grid image and occurring at the position of 1 cycle/mm. Therefore, when a visible image is reproduced from the image signal, a striped pattern having a spatial frequency of 1 cycle/mm occurs on the reproduced visible image.
Therefore, a novel method for generating a radiation image signal has been proposed in, for example, U.S. Pat. No. 5,028,784. With the proposed method, an analog image signal, which has been obtained from an image read-out operation carried out on a recording medium, such as X-ray film or a stimulable phosphor sheet, is sampled at sampling intervals smaller than such sampling intervals that a spatial frequency of aliasing caused to occur by a grid image coincides with a spatial frequency, which is the maximum of a desired spatial frequency range. The sampled image signal is digitized, and a digital original image signal is thereby obtained. Thereafter, the original image signal is subjected to filtering processing for reducing or eliminating the frequency components corresponding to the spatial frequency of the grid image. The original image signal, which has been obtained from the filtering processing, is then sampled at such sampling intervals that the spatial frequency, which is the maximum of the desired spatial frequency range, is set as a Nyquist frequency. In this manner, an image signal to be used in reproducing a visible radiation image is generated.
An image signal representing a radiation image is often subjected to compression processing such that as many image signals as possible can be stored (filed) on a storage medium, such as an optical disk, or such that the efficiency, with which the image signal is transmitted to a desired location, can be kept high. The compressed image signal, which has been obtained from compression processing, is then read from the storage medium or is transmitted to a desired location. Thereafter, the compressed image signal is subjected to decompression processing, and the resulting decompressed image signal is used during the reproduction of a visible radiation image.
Various methods for compressing an image signal have heretofore been used. Recently, various novel methods for compressing an image signal have been proposed in order to enhance the compressibility and to prevent the image quality (the sharpness) from becoming worse. For example, U.S. Pat. No. 4,941,194 discloses a method for compressing a radiation image signal by carrying out redundancy suppression encoding processing on an original image signal representing a radiation image, wherein the improvement comprises the steps of: carrying out pre-processing for decreasing the number of image signal components of the original image signal by an appropriate method, and subjecting the image signal, which has been obtained from the pre-processing, to the redundancy suppression encoding processing.
However, the problems have heretofore been encountered in that a moire often occurs on a visible radiation image reproduced from an image signal, which has been obtained from compression processing and decompression processing. The moire occurs particularly on a visible radiation image reproduced from a radiation image having been recorded by using a grid.
Therefore, a method for compressing an image signal, with which the occurrence of a moire can be prevented, has been proposed in, for example, U.S. Pat. No. 5,086,489. The proposed method for compressing an image signal comprises the steps of:
i) subjecting an original image signal, which is made up of a series of image signal components representing an image, to component number decreasing processing wherein: PA1 ii) classifying the new image signal components, which have been obtained from the component number decreasing processing, into main components, which have been sampled at appropriate sampling intervals, and interpolated components other than the main components, and PA1 iii) subjecting the interpolated components to interpolation prediction encoding processing based on the main components. PA1 the method for reproducing a radiation image comprising the steps of changing characteristics of the interpolation processing in accordance with conditions, under which the image signal before being subjected to the interpolation processing has been generated. PA1 wherein the improvement comprises the provision of a means for changing characteristics of the interpolation processing in accordance with conditions, under which the image signal before being subjected to the interpolation processing has been generated.
a) along each of a plurality of block lines which are set in parallel on an image, blocks each of which comprises a single picture element in the image are set at predetermined intervals, or blocks each of which comprises a plurality of picture elements in the image are set continuously or at predetermined intervals, the blocks being set such that the phases of the blocks located along a block line are shifted from the phases of the blocks located along a neighboring block line, PA2 b) a representative image signal component is determined for each block from the image signal component representing the single picture element in each block in cases where each block comprises the single picture element, or a representative image signal component is determined for each block from the image signal components representing the picture elements in each block in cases where each block comprises the plurality of the picture elements, and PA2 c) only the representative image signal components corresponding to the blocks are sampled as new image signal components,
The proposed method for compressing an image signal is very effective to prevent the occurrence of a moire but has the problems in that a comparatively long time is required to carry out the compression processing. Also, further improvements should be made in the compressibility and the image quality (the sharpness).
When a visible radiation image is reproduced from an image signal representing a radiation image, the reproduced image is often reduced or enlarged so as to satisfy the requirements of, in particular, diagnoses, restrictions by the image reproducing apparatus, or the like. For example, U.S. Pat. No. 4,568,973 discloses a technique for varying the magnification of reproduction in accordance with the size of a recording medium, such as a stimulable phosphor sheet, which was used during the image recording operation, in cases where the output size in an image reproducing apparatus is fixed.
Ordinarily, in cases where a radiation image represented by a digital image signal is to be enlarged, interpolation processing is carried out on the image signal, and an image signal representing an enlarged image is thereby formed. In cases where a radiation image is to be reduced, basically, the digital image signal may be thinned out. In general, in such cases, interpolation processing is carried out on the image signal, and then an image signal obtained from the interpolation processing is thinned out such that, for example, various scales of reduction may be achieved.
However, when a radiation image reduced or enlarged by carrying out the interpolation processing is reproduced, the problems occur in that a moire or an artifact occurs on the reproduced radiation image or the sharpness of the reproduced radiation image becomes markedly low. The moire occurs particularly on a visible radiation image reproduced from a radiation image having been recorded by using a grid. Also, the artifact occurs particularly when a radiation image is reproduced from an image signal, which has been obtained from irreversible compression processing.